376 J. M. Adams — Transmission of RonUjen Rays. 



more penetrating toward a second sheet of any other metal 

 than the original beam was. Experiments with the radiomi- 

 crometer have continued this view in most cases ; but the 

 effect of transmission through silver on penetrating power for 

 aluminium was found to be very small, while transmission 

 through aluminium appeared to decrease the penetrating 

 power of the rays for silver, so far as the deflections of the 

 instrument are an indication. This last result is contrary to a 

 conclusion reached by Walter* by a fluoroscopic method, and 

 throws some doubt upon the necessity of the transformation 

 theory which he proposes. In view of this doubt, direct 

 experimental evidence of transformation of one sort of ray 

 into another sort of ray in transmission through a metallic 

 sheet, was sought, with a negative result. The experiment 

 was performed by interposing in the path of the rays a plate 

 made of two sheets of different metals placed face to face. It 

 is to be expected that if transformation occurs in the metals, 

 the effect of the second metal upon rays transformed by the 

 first will not be quantitatively the same as the effect of the first 

 metal upon rays transformed by the second, when the order 

 of the two sheets in the plate is reversed. In other words, 

 the transformation theory leads us to expect that the effect of 

 a two-piece plate upon the deflection of the radiomicrometer 

 will depend upon the order of the pieces. The absence of 

 such dependence was shown by experiment, and is evidence of 

 the absence of transformation. 



With the probability of transformation in transmission thus 

 removed, and with experimental evidence showing that any 

 effect of the surfaces of the metal upon transmission is small, 

 the only conceivable action produced upon a beam of rays by 

 transmission through a metallic sheet is an absorbing action. 

 To explain the phenomenon observed with different thicknesses 

 of the same metal, we must suppose, as Rontgenf has sug- 

 gested, that the rays from a tube are heterogeneous, and that 

 different kinds of rays are differently absorbed in any one 

 metal. To explain the apparent reduction of penetrating 

 power by transmission in certain cases, we must suppose that 

 rays which are more penetrating for some metals are less pene- 

 trating for others, — that is, that metals show relatively selec- 

 tive absorption,^: and that the apparent reduction of penetrating 

 power by transmission through silver, etc., is a real reduction 

 of penetrating power with respect to aluminium. In judging 

 of the significance of Walter's fluoroscopic work, as in fact of 

 all work on Rontgen rays, it must be borne in mind that the 



* B. Walter, Ann. d. Phys., xvii, p. 561, 1905. 



f W. C. Kontgen, Ann. d. Phys., lxiv, p. 18, 1898. 



% J. M. Adams, this Journal, xxiii, p. 91, 1907. 



